In the atmospheric sciences, many types of aircraft expendable instruments are ejected from altitude to provide in situ measurements. For example, aircraft expendable instruments may include: dropsondes, Airborne eXpendable BathyThermographs (AXBT), Airborne eXpendable Current Profilers (AXCP), Airborne eXpendable Conductivity Temperature and Depth probes (AXCTD), and drifting buoys, in addition to others. Aircraft expendable instruments may be launched or ejected from an aircraft using a launch tube. A launch tube offers a passage from the inside of a cabin or internal structure of an aircraft to the outside environment.
A “dropsonde,” also known as a dropwindsonde or a sonde, is an example aircraft expendable instrument. A dropsonde is a weather instrument that takes measurements as it descends through the atmosphere to the surface of the Earth. A dropsonde may include a radio transmitter, pressure, humidity, and temperature sensors, a microprocessor, a battery, and a parachute, in addition to other components. A dropsonde continuously measures atmospheric properties such as temperature, humidity, and wind profiles as it descends to the surface, transmitting those measurements back to the launching aircraft via radio waves. Dropsondes provide a detailed profile of atmospheric properties that is of great value for weather prediction and climate research.
Detecting precisely when a dropsonde is launched from the aircraft can be very important part of operating the dropsonde device to produce high quality atmospheric measurements. For example, it can be important to accurately time critical operations after launch, such as when to activate release of the parachute that will be used to slow the descent of the dropsonde, and when to modify the RF power level of the dropsonde transmitter after launch. Moreover, a precise launch time may be needed to provide accurate data products using the dropsonde scientific instruments.
Prior aircraft expendable instrument launch systems fail to determine a precise launch time. In the example of dropsonde launch detection systems, designs rely on integrated launch detection and parachute release functions. When the dropsonde is launched from an aircraft, the parachute is initially packed under a parachute cap secured to the dropsonde housing with a ribbon wrapped around the dropsonde body holding on a parachute cap. Before deployment, the parachute blocks light from reaching a Cadmium Sulphide (CdS) photo detector positioned in the parachute compartment from either the sun or from an LED installed within the parachute compartment. After launch and parachute deployment however, light provided from either the sun or an LED may reach the photo detector to signal that the launch has occurred.
The prior launch technology is problematic because it requires the success of a series steps in order for launch detect to work correctly. For example, the steps required to launch a dropsonde include: 1) the unwinding of the ribbon wrapped around the outside of the dropsonde body immediately after launch, 2) the release of the parachute cap from the dropsonde tube body, 3) the ejection of the parachute from its compartment, and 4) the detection of light by the CdS detector after parachute release. Even if all of the steps execute properly, performance of several dropsonde components must perform within strict limits or the dropsonde launch may not be properly detected. Possible complications include variable CdS detector sensitivity and variable dropsonde tube opacity to sunlight. In addition, if the parachute is not properly packed in its compartment, blocking all light to the CdS detector prior to launch, the CdS detector will fail to detect the launch of the dropsonde.
What is needed is an aircraft expendable instrument launch detection system that is simpler, requires fewer steps, fewer parts, provides increased reliability, and is capable of providing a precise launch time.